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ELECTRONIC STRUCTURE OF SURFACES, SURFACE MAGNETISM AND SURFACE PHASE
TRANSITIONS
A. Freeman, Chen Wang, H. Krakauer, M. Posternak
To cite this version:
A. Freeman, Chen Wang, H. Krakauer, M. Posternak. ELECTRONIC STRUCTURE OF SURFACES,
SURFACE MAGNETISM AND SURFACE PHASE TRANSITIONS. Journal de Physique Colloques,
1980, 41 (C1), pp.C1-39-C1-41. �10.1051/jphyscol:1980107�. �jpa-00219577�
JOURNAL DE PHYSIQUE Colloque
Cl ,
supplkment au n O 1, Tome 41, janvier 1980, page C1-39ELECTRONIC STRUCNRE OF SURFACES I SURFACE MAGNETISM AND SURFACE PHASE TRANSITIONS
*
A. J. Freeman, C.S. wang: H. Kxakauer and M. Posternak
Physics Department and the MateriaZs Research Center, Northestern University, Evanston, IL 60201 U.S.A.
Abstract.- We describe and discuss several recent developments in the study of surface phenomena in this rapidly developing area of research which is becoming of great interest to the Mtissbauer spectroscopist.
The study of surface phenomena has added a Similarly, sophisticated theoretical methods new and important area of research which has yet are being developed for accurately determining the to be exploited fully by the g6ssbauer spectro- electronic structure of transition metal surfaces.
scopist. As discussed at the Conference, this is These use the bulk energy band methods in the local particularly true as regards transition metals density formalism as extended to the case of a where recent advances in experimental and theore- thin film 13- 151. As in bulk systems, a number of
tical methods for studying bulk and surface thin film studies for transition metals has shown properties have increased our knowledge about that self-consistency plays an essential role these phenomena but have also raised important [ 6-12,153.
questions about our fundamental understanding in I discuss in detail the work which we r.9-121 a dramatic way. Here I will mention two areas,
surface magnetism and surface phase transitions, which are particularly important examples of recent developments in this fast growing field.
A. Surface States. Surface Magnetization and Electron Spin Polarization in Ferromagnetic Transition Metals.
A striking example of some of the con£ licting results is the contradictory conclusions about the validity of the Stoner-Wohlfarth-Slater band theory of ferromagnetism drawn from electron spin polarization experiments [I]. The possibly important role of surface phenomena in reconcil- ing these difficulties has been recognized in
have carried out on ferromagnetic transition metal films using our recent extension of the local spin density functional formalism to thin films. These are the first spin polarized ab initio self-consis- tent energy band studies of ferromagnetic transi- tion metal filme that are thick enough C9-layers for Ni(001) and 7 layers for Fe(001)l to accurately determine the energy dispersion and spatial charac- ter of the surface states and their effects on the surface spin polarization layer by layer spin and charge distributions and layer projected density of states.
One striking feature of the results is the Friedel type oscillation induced by the surface recent years and theoretical efforts at describing discontinuity in the layer spin magnetic density the electronic structure of surfaces have intensi- but not in the charge density. A result of fied. These investigations share the common particular interest here is the prediction of a problem with bulk studies of treating localized decrease (by 2077) in the surface layer magnetic d electrons along with the itinerant s-p electrons. moment in Ni and a larger increase in the case of For bulk systems, considerable progress has been Fe. There is no evidence for magnetically 'dead' made in the last few years in this direction 123. layers.
t This work was supported by the National Science Foundation (Grant No.DMR 77-23776) and under the NSF-MRL program through the Materials Research Center of Northwestern University (Grant No. DMR 76-80847), by the Air Force Office of Scientific Research (Grant No. 76-2948) and the Department of Energy.
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Present address :Physics Department, University of Maryland, College Park, MD 20742 U.S.A.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980107
C
1-40 JOURNAL DE PHYSIQUEB. E l e c t r o n i c a l l v Driven S u r f a c e Phase T r a n s i t i o n I n t e r e s t i n s u r f a c e phase t r a n s i t i o n s - a n d t h e r e l a t i v e r o l e s of phonons and e l e c t r o n i c s t r u c t u r e - have r e c e n t l y i n t e n s i f i e d . I n a n important case,
l o w - e n e r g y - e l e c t r o n - d i f f r a c t i o n (LEED) s t u d i e s of t h e tungsten C16-181 (001) s u r f a c e have found a temperature dependent phase t r a n s i t i o n Cc(2x2)
, . .
s t r u c t u r e ] when t h e temperature i s lowered below about 300 K. This t r a n s f o r m a t i o n seems t o be of second o r d e r and i s r e v e r s i b l e on varying o n l y t h e temperature. A s i m i l a r t r a n s i t i o n has been observed on Mo(001) [17]. Recent i n v e s t i g a t i o n s [17,18] conclude t h a t no chemisorbed impurity ( i n c l u d i n g hydrogen) i s p r e s e n t on t h e s u r f a c e when t h e low temperature ~ ( 2 x 2 ) s t r u c t u r e i s observed, thus implying t h a t t h e t r a n s i t i o n i s c h a r a c t e r i s t i c of t h e c l e a n s u r f a c e . The i n t e r - p r e t a t i o n o f t h i s phase t r a n s i t i o n has l e d t o c o n t r o v e r s y about i t s o r i g i n and has centered about t h e p o s s i b l e formation and r o l e [19] of a s u r f a c e charge d e n s i t y wave (CDW) a s i n t h e
layered t r a n s i t i o n m e t a l dichalcogenides [20].
A t t h e Conference, I d i s c u s s s u r f a c e phase t r a n s i t i o n s i n W(001) u s i n g t h e r e s u l t s of r e c e n t d e t a i l e d s t u d i e s whichawe have c a r r i e d o u t u s i n g t h e s e l f - c o n s i s t e n t LAPW method f o r t h i n f i l m s [13-15,211. I n p a r t i c u l a r , we have c a l c u l a t e d t h e l a y e r - p r o j e c t e d ~ ( q ) + [21], i n c l u d i n g approximate m a t r i x elements, f o r W(001) u s i n g our s e l f - c o n s i s t e n t (SC) e l e c t r o n i c e n e r g i e s and wave f u n c t i o n s o b t a i n e d f o r a 7-layer f i l m [15]. Our c a l c u l a t i o n s r e v e a l a l a r g e and prominent peak i n x(;) a t t h e zone boundary ~ [ & ( 1 , 1 ) 1
Now, t h e e l e c t r o n i c response t o p e r i o d i c atomic d i s p l a c e - ments w i t h wave v e c t o r
+
q i s determined by t h e g e n e r a l i z e d response f u n c t i o n X(q), and a n i n s t a -+
b i l i t y i n t h e ground s t a t e can s e t i n i f x(;) i s
l a r g e [22]. Thus our r e s u l t s provide s t r o n g t h e o r e t i c a l evidence i n d i c a t i n g t h a t t h e phase t r a n s i t i o n i s indeed e l e c t r o n i c a l l y d r i v e n i n agreement w i t h t h e CDW mechanism. Our r e s u l t s a r e shown t o be c o n s i s t e n t n o t only w i t h t h e p a r a l l e l s h i f t model on c l e a n W(001) [23,24] b u t a l s o w i t h r e s u l t s concerning W(001)c(2x2)-H a t room
.. . -
temperature where i t has been suggested t h a t t h e s u p e r s t r u c t u r e involves displacements o f W atoms
( r e c o n s t r u c t i o n of t h e s u b s t r a t e ) and t h a t t h e r o l e o f hydrogen i s one of impurity s t a b i l i z e r [25].
When a s t r u c t u r a l phase t r a n s f o r m a t i o n proceeds v i a " s o f t mode" i n s t a b i l i t y , i t i s p o s s i b l e t o r e p r e s e n t t h e s t a t i c d i s t o r t i o n s c h a r a c t e r i z i n g t h e s t r u c t u r a l rearrangements by a s e t o f 'If rozen i n " phonon-like displacements. As T o s a t t i 1191 has noted, t h e p a r a l l e l s h i f t model 123) e x a c t l y f i t s and M5(ll) p o l a r i z e d phonon.
This i s p r e c i s e l y t h e v e c t o r f o r which our d e t e r m i n a t i o n of X shows a peak a t
M.
The conduc- t i o n e l e c t r o n response f u n c t i o n of t h e unrecon- s t r u c t e d W(001) s u r f a c e t h u s r e v e a l s a p o s s i b l e i n s t a b i l i t y a t t h e zone boundaryM,
and t h i s s u p p o r t s t h e CDW mechanism f o r t h e phase t r a n s i t i o n i n which t h e o n s e t of 2D-CDW must be screened by an overdamping o f t h e corresponding phonon mode, l e a d i n g t o t h e p a r a l l e l s h i f t model. We n o t e t h a t t h e n e s t i n g o f t h e s u r f a c e resonance s t a t e s r e s p o n s i b l e f o r t h e peak i nx
appears i n t h i s c a l c u l a t i o n although t h e s p i n - o r b i t i n t e r a c t i o n has been n e g l e c t e d . The occurrence of s p i n - o r b i t gaps is t h u s n o t r e q u i r e d t o e x p l a i n t h e recon- s t r u c t i o n . An i n s t a b i l i t y w i t h r e s p e c t to CDW formation might a l s o e x p l a i n why hydrogen could a c t a s a n impurity s t a b i l i z . e r i n t h e formation o f W(001)c(2x2)-H a t room temperature. A s notedabove, i t has been suggested [25] t h a t t h e s u p e r s t r u c t u r e involves r e c o n s t r u c t i o n of t h e W
s u b s t i t u t e . This could occur i f t h e i n t e r a c t i o n between t h e H atoms and t h e CDW s t a b i l i z e s t h e e x c i t o n i c ground s t a t e a t higher temperatures by
t h e a d d i t i o n of a "pinning" term i n t h e Landau free-energy.
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